Align edges and join/merge vertices without adding additional faces etc
You can use the snap to vertex functions, with the mode set to 'active', like this:



The first button marked with a red arrow activates snapping (however, you can use the Ctrl key as a much faster alternative. The second one allows you to set the snapping target, in this case vertices. The third one tells Blender to try to snap the selection based on the last selected vertex, the so-called active vertex (red circle). You can identify it by its color, in my theme white as opposed to orange.

Now, activate grab and move G and move the selection towards the target point. When you're somewhat close, the mesh should snap there entirely. Use 'remove doubles' from the 'Tools' Panel to eliminate overlapping vertices.